Erbium doped fiber amplifiers (EDFAs) are used to amplify signals passing through an optical fiber. EDFAs rely on a pump laser to excite erbium atoms doping several meters of optical fiber. When a light signal passes through the excited doped fiber, the erbium reverts to its unexcited energy state and gives up the pump energy as a photon of the same wavelength as the light signal triggering the reversion.
EDFAs, unfortunately, have at least two undesirable characteristics. The first undesirable characteristic is that the gain of the EDFA is not flat across the range of wavelengths used in optical communication fibers. This gain skew results in some of the channels of a dense wavelength division multiplexing (DWDM) optical signal receiving more amplification than other signals.
The second undesirable characteristic is that the gain imparted to the various signals in a DWDM optical signal depends on the initial magnitude of the signal—strong signals receive more amplification than weak signals.
The problems caused by these characteristics build up as more EDFAs are used in a signal path. Since EDFA repeaters are used approximately every 60 km, long haul signals are amplified many times. If the gain characteristics are not compensated, the weak signals will be extinguished as the stronger signals consume the amplification power.
Existing systems compensate the signal path to ensure the weak signals are not extinguished. Generally, the path compensation is achieved by placing static filters in the signal path to equalize or set the power of the various channels. Once equalized, the signals may be amplified by a series of EDFAs—as long as the signal do not change power relative to each other. While this enables the use of EDFAs for long haul communication channels, the resulting long haul networks are unable to dynamically respond to the addition or deletion of various signals in the fiber. What is needed is a dynamic equalization method and apparatus.